At present, one representative radiation diagnosis system is an X-ray CT system. With the advent of helical scanning, the clinical significance of high-speed scanning has been made clear. Nowadays, for further speedup, a helical scan is usually performed with a multislice CT (also called a multiple-row CT or multi detector row CT (MDCT)). For details on multislice CTs and helical scanning, see, for example, non-patent documents 1, 2, and 3.
The X-ray measuring system (also referred to simply as “measuring system”) of the scanner of a multislice CT is illustrated in FIG. 1 by way of example. A plurality of X-ray detector elements (also referred to simply as “detector elements”) 101 are arranged in a fan-angle (φ) direction in FIG. 1 and form one row, and furthermore, a plurality of rows of detector elements are arranged in a Z-axis direction crossing the fan-angle direction, thereby forming a multiple-row detector 100. Therefore, it is also sometimes called a multiple-row CT. In FIG. 1 the X-ray detector elements in the Z-axis direction are 12 rows, but in actuality, they are more than tens of rows.
Although not illustrated in FIG. 1, there is a data acquisition system (DAS) for measuring an analog signal from the detector element 101 and converting the analog signal into a digital signal. A number of DASs are provided according to the number of rows of detector elements 101, but the number of DASs is normally smaller than the number of rows of detector elements 101 (e.g. in the example of FIG. 1, 4 DASs, compared with 12 rows). In the case of multislice CTs, the number of rows is called in the number of DASs, not the number of rows of detector elements 101. That is, if there are four DASs, the corresponding CT is called a 4-row multislice CT.
In FIG. 1, an X-ray source 200 radiates X-rays with a cone angle (X-ray beam thickness) which covers 8 rows of detector elements 101, through a slit (collimator) 300. DAS, for example, can bundle two rows of detector elements 101 together, process it as one row, and handle 4 rows of X-ray measurement data in total. That is, in this case, practically two rows of detector elements 101 function as one row of detector elements having double length in the Z-axis direction. For such bundling, there are cases where no bundling is performed depending upon inspection purposes, or 4 rows or 8 rows of detector elements 101 can be freely bundled.
One row made by bundling rows forms one row of detector elements, so the “detector element row” used in the following description, unless otherwise specified, does not mean rows each of which is formed of detector elements 101, but does mean a detector element row made by bundling rows of detector elements. Therefore, for instance, “the arrangement pitch in the Z-axis direction of detector element rows” means the arrangement pitch in the Z-axis direction of data that are handled by DAS. If two rows are bundled, the arrangement pitch in the Z-axis direction of detector element rows is twice the pitch between the detector element rows actually arranged in the detector 100.
Note that the X-ray detector element 101 outputs an electrical signal proportional to the intensity of X-rays transmitted through a radiography subject. The electrical signal is measured by DAS. In the following description, the data thus obtained is sometimes referred to as X-ray detection data or X-ray data.
In addition, projection data are data obtained by integrating the X-ray attenuation coefficients of a test subject along a path of X-rays connecting the X-ray source 200 and each detection element 101. The projection data are obtained by suitably converting the X-ray measurement data output of DAS. If, in a certain projection angle direction (rotational angle of the measuring system of FIG. 1), projection data are acquired over the range in which the detector elements 101 are arranged in the fan angle (φ) direction, and the data acquisition is performed through one rotation, then enough projection data to reconstruct an image are obtained (Note that one rotation is not always needed. It is sufficient if the data acquisition is performed in a rotational angle range sufficiently greater than half a rotation). This is the principle of CT. Multislice CTs are designed to simultaneously perform this acquisition of projection data at a plurality of Z-axis direction positions.
Helical scanning is a scanning method in which, while this measuring system is continuously rotating around a test subject (or a photography subject), the test subject moves at a fixed speed in the Z-axis (test subject axis or rotation center axis) direction. That is, as schematically illustrated in FIG. 2, helical scanning is high-speed scanning such that the measuring system describes a spiral orbit with respect to a test subject 400.
It is well known that if a helical scan is performed with a multislice CT, artifacts (abnormal patterns) occur in an image. Because of its shape, the artifact is called a windmill artifact. This artifact occurs primarily at a place where the structure of a photography subject changes sharply. The name of the windmill derives from an image illustrated in FIG. 3.
The image illustrated in FIG. 3 was obtained by scanning 17 spheres (radius 15 mm) with a high X-ray attenuation coefficient (CT value 2000) disposed one at the center of rotation and respectively four at each of positions of radii 100 mm, 140 mm, 180 mm, and 220 mm, and performing image reconstruction with the outer peripheral portion (position of Z=14 mm with the center of a sphere at the center of rotation as zero (Z=0 mm)) of each sphere as an image reconstructing plane (imaging position). Although 17 white small circular regions should be obtained as a correct artifact-free image, black and white streaks in the shape of a windmill have occurred around the spheres. These are windmill artifacts.
As the same photography subjects and helical scanning conditions as those of FIGS. 3 and 4 will be employed later to show advantages of the present invention, a brief description will be given below.
The helical scanning conditions, as illustrated in FIG. 3, are “16-row multislice CT, scan slice thickness 1 mm, helical pitch 13 (beam pitch 0.8125), field of view 500 mmφ, and 900 views/rotation.”
Since, in FIG. 1, it is assumed that there are 4 DASs, the X-ray beam is partitioned into four sheets. Therefore, in the case of a 16-row multislice CT, there are 16 DASs, so the X-ray beam is partitioned into 16 sheets.
The “scan slice thickness 1 mm” means that the thickness of each of the four X-ray beam sheets partitioned is 1 mm at the center of rotation. In the case where detector element rows are arranged in the Z-axis direction with a finer pitch than that of DAS, each of the partitioned X-ray beams is formed by bundling of a number of detector element rows.
The “helical pitch 13” means that the photography subject moves in the Z-axis direction by 13 times the scan slice thickness, i.e. 13 mm while the scanning system makes one rotation. Likewise, the “beam pitch 0.8125” means that the photography subject moves in the Z-axis direction by 0.8125 times the overall thickness of the X-ray beam at the center of rotation during one rotation.
The image reconstruction method used herein is a method called TCOT (True Cone beam Tomography reconstruction algorithm), a detailed description of the theory being disclosed, for example, in patent document 2 and non-patent document 4 described later. The image reconstruction method of the present invention is explained by changes in existing reconstruction methods and resultant images. The present invention can select any of a wide variety of image reconstruction methods that can be employed in the helical scanning of multislice CTs, but the reason that TCOT is employed in this specification is as follows.
First, TCOT is a simple method in which projection data with a cone angle (see FIG. 1) are back projected accurately in accordance with the cone angle without approximation, so the explanation of the image reconstruction method of the present invention becomes easy. In addition, performing back projection in accordance with a cone angle is to perform three-dimensional back projection, so the image reconstruction method of the present invention can be very directly applied.
Second, if other image reconstruction methods that have been put to practical use are employed, different kinds of artifacts not handled by the present invention will contaminate an image, but this image reconstruction method reduces the problem and therefore it becomes easy to clearly show advantages of the present invention with an image.
Although the expression “projection data are back projected” is employed, in the image reconstruction by CT, projection data are not back projected as they are. Data obtained by performing a convolution operation or a filtering process on projection data is called convolution corrected projection data or filter corrected projection data, and these corrected projection data are back projected when performing image reconstruction of CT. In performing back projection, it is also necessary to perform an interpolation process on corrected projection data. These will be apparent to those skilled in the art, so in this specification, back projection including these processes is simply expressed as “projection data are back projected.”
It is well-known to those skilled in the art that windmill artifacts occur when a sampling pitch in the Z-axis direction is not sufficiently fine. The sampling pitch in the Z-axis direction, as illustrated in FIG. 5, is determined by the arrangement pitch in the Z-axis direction of the detector element rows (each of which is bundled by one of the DASs). Note that FIG. 5 is equivalent to a figure viewed from a side direction crossing the Z-axis direction of the measuring system illustrated in FIG. 1. Directing attention to some of the detector element rows illuminated with X-rays from the X-ray source 200, they are enlarged in the Z-axis direction.
As illustrated in FIG. 5, a line connecting the X-ray source 200 (focus of the X-ray tube) and the detector-element aperture center represents the position of projection data of each row. That is to say, data sampling is performed along this line. At this time, s(r) represents a sampling pitch in the Z-axis direction and depends upon a distance from the Z-axis (a plus sign is taken on the side of the X-ray source 200). A value at the rotation center (iso center) axis (Z-axis) is represented by siso.
This siso is the same value as the scan slice thickness because it is normally discussed on the assumption that the detector-element aperture width in the Z-axis direction is the same as the Z-axis direction arrangement pitch. In the following description, the scan slice thickness is assumed to be equal to siso. In CT, projection data are only discretely obtained in other directions as well as the Z-axis direction, but since in the following description a discussion is concentrated about the Z-axis direction, unless other specified, the expression “sampling pitch” means a sampling pitch in the Z-axis direction.
The values of projection data at a position between spots where data are being sampled must be obtained by interpolating the projection data obtained by neighboring rows of detector elements. If s(r) (also sometimes referred to simply as s) is sufficiently small, the interpolation result is able to correctly reflect changes in the Z-axis direction of a photography subject, i.e., it becomes a value very close to the true projection data at the intermediate position, so there is no problem. If the sampling pitch s is great, the interpolation result becomes a value away from the true projection data, which can cause artifacts. These artifacts are windmill artifacts, but the reason they show windmill-shaped patterns is omitted here because it will be explained together with helical motion.
If a scan slice is made thick, then the sampling pitch s becomes wide and therefore windmill artifacts occur significantly. In the present CTs, the smallest value of the scan slice thickness siso is about 0.5 to 0.6 mm. This thickness causes not a great problem, but windmill artifacts occur to the degree illustrated in FIGS. 7(a) and 7(b). In ordinary CT examination, because of examination efficiency (if a scan slice is made thin, it takes time to scan a predetermined Z-axis direction range) and a reduction in image noise (if thin, image noise becomes great), CT is operated with a scan slice thickness of 1 mm or greater, not the thinnest slice. In that case, windmill artifacts are further increased.
If interference with diagnosis by windmill artifacts is to be prevented, a thick image must be made. That is, helical scanning makes it possible to make an image far thicker than the scan slice thickness siso by use of a large number of projection data away in the Z-axis direction, so that windmill artifacts are averaged and reduced.
If windmill artifacts are to be suppressed to the degree that they are not seen enough, the thickness of an image must be made two or more times a scan slice thickness empirically, although it is a matter of degree. However, in this case, spatial resolution in the Z-axis direction is often insufficient, so this also interferes with clinical diagnosis. After all, an operator needs to determine a trade-off between windmill artifacts and Z-direction spatial resolution to obtain a desired image.
Thus, the problem of windmill artifacts is particularly important. If this is solved, then the possibility of an erroneous diagnosis and clinical difficulty due to artifacts is reduced, inspection time is shortened, and higher definition images are obtained, so that the value of CT diagnosis is enhanced.
As the latest technique to alleviate the windmill problem, that is, the problem of a sampling pitch being coarse, there is a “z-flying focal spot” method disclosed in non-patent document 5 by way of example. The “z-flying focal spot” will hereinafter be referred to as zFFS.
This method, for instance, as illustrated in FIG. 6, is one in which a focal position is alternately moved (back and forth) in the Z-axis direction during a scan. In CT, projection data are acquired, for example, in 1000 directions, i.e., for each 0.36 degrees, one set of projection data obtained at one angle being called a view. That is, for each view, in other words, between odd and even views, focal positions are interchanged. Such interchanging is called flying. This focus flying width (flying distance) is set such that, for example, αiso illustrated in FIG. 6 becomes ¼.
As a result, as illustrated in FIG. 6, paths (see dotted lines) of projection data at odd and even views are positioned so that they are alternately threaded through the intervals. In FIG. 6, a solid line represents a projection data position (path) when the zFFS method is not applied, that is, a focal position is not moved, and a (r) represents a distance between a first projection data position when the zFFS method is not applied and a second projection data position when the zFFS method is applied. In addition, RF represents a distance between the center of rotation and the focus, and RFD represents a distance from the focus to the detector-element row (detector-element aperture center).
If a difference in projection angle between adjacent views is neglected, that is, if a difference in projection data due to a difference in projection angle direction is approximated to be almost negligible, when odd and even views are added together, the interval between dotted lines of FIG. 6 is one-half the scan slice thickness siso at the center of rotation. Thus, the sampling pitch has become fine.
As a result, images having windmill artifacts suppressed can be obtained as illustrated in FIGS. 7(b) and 7(d). In a clinical image illustrated in FIG. 7(c) streaks flowing downward from the top bone structure are windmill artifacts. In a simulation image illustrated in FIG. 7(a), streaks occurring from the right protrusion, and streaks occurring from the edge of the top cavity, are windmill artifacts. These are improved as illustrated in FIGS. 7(d) and 7(b) by the zFFS method.
The images illustrated in FIGS. 7(b) and 7(d) are obtained by the image reconstruction method of non-patent document 5, and although approximation errors occur, projection data for each view are handled as being acquired along a path indicated with a dotted line in FIG. 6. Therefore, even in the case of the image reconstruction method disclosed in non-patent document 5, back-projection is basically performed along a path where projection data were acquired.
With the conditions previously described in FIGS. 3 and 4, a simulation image obtained when the zFFS method is applied is illustrated in FIG. 8. Windmill artifacts have been suppressed over the whole region. However, windmill artifacts near the center of the visual field are satisfactorily reduced, but windmill-artifact suppression becomes smaller as spheres are positioned away from the center. Particularly, in the top portion of the image, the suppression effect is extremely small. In addition, except the image center portion, feeble streaks appear as straight fine lines, and particularly in the lower left portion, shower-shaped or striped artifacts become predominant. These will be described later.
The image reconstruction method used herein, that is, the reconstruction method by which an image illustrated in FIG. 8 was obtained is not completely the same as the method in non-patent document 5, but even if the same method as non-patent document 5 is employed, results better than the result illustrated in FIG. 8 cannot be obtained. In all of the existing image reconstruction methods, back projection is commonly performed along a path where projection data were acquired. A difference resides only in that in the case of non-patent document 5 slight approximation errors occur, while the image reconstruction method used herein does not cause any approximation error. As described later, the particularly important feature of the present invention resides in a difference between the case of performing back projection along a path in which projection data were obtained, and the case of performing back projection along an entirely different path from that path.
Note that patent document 1 and non-patent document 6 disclose techniques for eliminating fine shower-shaped artifacts, so-called aliasing artifacts, by suitably handling unequal-interval sampling data in an imaging plane, not a Z-axis direction.
Patent document 1: Japanese patent laid-open publication No. 2005-40236.
Patent document 2: U.S. Pat. No. 5,825,842.
Non-patent document 1: W. Kalender, “Computed Tomography,” Publicis Corporate Publishing, Erlangen (2005).
Non-patent document 2: Y. Nobuta, “CT system advancing speedup,” online, Toshiba Review Vol. 57, No. 2 (2002), Jun. 7, 2006 retrieval, Internet <URL:http://www.toshiba.co.jp/tech/review/2002/02/57—02pdf/a03.adf>.
Non-patent document 3: K. Tsujioka, “Equipment Engineering (4) of X-Ray CT System—Development of Multislice CT—,” online, Japanese Radiation Technical Society Journal Vol. 58, No. 5 (May 2002), Jun. 7, 2006 retrieval, Internet <URL:http://www.nv-med.com/jsrt/pdf/2002/58—5/651.pdf>.
Non-patent document 4: M. Silver, K. Taguchi, K. Han, “Field of view dependent helical pitch in cone-beam CT,” Proc. SPIE 4320, 839-850 (2001), San Diego, Calif., U.S.A.
Non-patent document 5: T. Flohr et. al, “Image reconstruction and image quality evaluation for a 64-slice CT scanner with z-flying focal spot,” Medical Physics 32 (8) page 2536-2547 (August 2005).
Non-patent document 6: I. Mori et. al, “Alleviation of aliasing artifact in CT,” Medical Imaging Technology Vol. 21, No. 4, September 2003.